Tumor-restricted surface antigens may be targets for diagnosis and immune-based therapies. Monoclonal antibody 8H9 is a murine IgG1 hybridoma derived from the fusion of mouse myeloma SP2/0 cells and splenic lymphocytes from BALB/c mice immunized with human neuroblastoma. By immunohistochemistry, 8H9 was highly reactive with human brain tumors, childhood sarcomas, neuroblastomas and less so with adenocarcinomas. Among primary brain tumors, 15/17 glioblastomas, 3/4 mixed gliomas, 4/11 oligodendrogliomas, 6/8 astrocytomas, 2/2 meningiomas, 3/3 schwannomas, 2/2 medulloblastomas, 1/1 neurofibroma, 1/2 neuronoglial tumors, 2/3 ependymomas and 1/1 pineoblastoma were tested positive. Among sarcomas, 21/21 Ewing's/PNET, 28/29 rhabdomyosarcoma, 28/29 osteosarcomas, 35/37 desmoplastic small round cell tumors, 2/3 synovial sarcomas, 4/4 leiomyosarcomas, 1/1 malignant fibrous histiocytoma and 2/2 undifferentiated sarcomas tested positive with 8H9. 87/90 neuroblastomas, 12/16 melanomas, 3/4 hepatoblastomas, 7/8 Wilm's tumors, 3/3 rhabdoid tumors and 12/27 adenocarcinomas also tested positive. In contrast 8H9 was nonreactive with normal human tissues including bone marrow, colon, stomach, heart, lung, muscle, thyroid, testes, pancreas, and human brain (frontal lobe, cerebellum, pons and spinal cord). Reactivity with normal cynomolgus monkey tissue was similarly restricted. Indirect immunofluorescence localized the antigen recognized by 8H9 to the cell membrane. The antigen is proteinase-sensitive and is not easily modulated off cell surface. 8H9 immuno-precipitated a 58 kD band following N-glycanase treatment, most likely a protein with heterogeneous degree of glycosylation. This novel antibody-antigen system may have potential for tumor targeting.
Monoclonal antibodies such as 3F8 (1) and 14.18 (2) against GD2 in neuroblastoma, M195 against CD33 in acute leukemia (3), anti-HER2 antibodies in breast cancer (4) and anti-CD20 antibodies in lymphoma (5) have shown efficacy in recent clinical trials. The prognosis in glial brain tumors and metastatic mesenchymal and neuroectodermal tumors remains dismal despite innovations in chemotherapy and radiation therapy. Immunotherapy may offer new possibilities for improving the outcome in these patients.
Tumor antigens expressed on cell membrane are potential targets in immunotherapy. Examples of tumor antigens expressed on glial tumors include neural cell adhesion molecules (6), gangliosides such as GD2 and GM2 (7), and neurohematopoeitic antigens (8). Recent investigations have focused on growth factor receptors as immune targets, in particular type III mutant epidermal growth factor receptor (EGFRvIII) which has been shown to be expressed on 50% of glial brain tumors (9). Notwithstanding the universal expression of NCAM by neuronal cells, two clinical studies have utilized anti-NCAM antibodies in patients. MAb UJ13A was shown to accumulate in gliomas by virtue of disruption of blood brain barrier locally (10) and another antibody, ERIC-1 was used in a therapeutic setting in resected glioma cavities with some clinical benefit (11).
Recent studies have targeted immunotherapy to extracellular matrix around tumor cells. Tenascin has been reported to be expressed in 50-95% of glial brain tumors as well as on mesenchymal tumors, carcinomas and normal human glial, liver and kidney cells (12). Anti-tenascin monoclonal antibodies 81C6 (13) and BC-2 and BC-4 (14) administered intra-cavity have recently been reported to show efficacy in the treatment of patients with malignant gliomas. However, since these antigens are also present to varying degrees on normal human neural and non-neural cells, their clinical utility would depend on their overexpression by brain tumors when compared to normal tissues. With the exception of EGFRvIII, the glial tumors antigens described to date are generally found on normal brain tissue, or are restricted to intracellular compartments, thus with limited clinical utility for antibody targeting.
Membrane antigens that have been targeted on osteosarcoma include GD2 (15), CD55 (16) and an as yet undefined osteosarcoma-associated antigen recognized by the MoAbs TP-1 and TP-3 (17). However, these antigens are present to varying degrees on normal tissues. Similarly the glycoprotein p30/32 coded by the MIC2 oncogene and recognized by the monoclonal antibody O13 in the Ewing's family of tumors is expressed on normal tissues (18). In rhabdomyosarcoma, the MyoD family of oncofetal proteins is nuclear in localization (19) and therefore inaccessible to antibody-targeted immunotherapy.
An ideal tumor antigen for targeted immunotherapy should be absent on normal tissues and abundantly expressed on tumor cell surface. Such tumor-specific antigens e.g. idiotypes in B cell lymphoma are rare (20). Moreover, a “generic” tumor-specific antigen expressed on tumor cells of varying lineage recognized by monoclonal antibodies may have broader utility in antibody-based strategies. We describe here a novel tumor-associated antigen, recognized by a murine monoclonal antibody 8H9, expressed on cell membranes of a broad spectrum of tumors of neuroectodermal, mesenchymal and epithelial origin, with restricted distribution on normal tissues.